1. Field
This invention relates generally to magnetic disk data storage systems, and more specifically to a system and method of encoding and decoding a plurality of bits of information within a single write current pulse that is recorded in a bit cell on a magnetic disk medium.
2. Prior Art
The continuing trend in magnetic disk data storage systems such as computer hard disk drives (HDDs) is toward smaller, faster and less costly devices with ever-increasing capacity. The remarkable increase in the processing ability of computers in recent years has given rise to sophisticated software programs that require large capacity HDDs to store them.
In addition, disk capacity is increasingly challenged by large video, data, and music files that are now common, and the advent of holographic image files will demand significantly more disk space yet again.
In general, HDDs are comprised of one or more rotating disks with a plurality of evenly-spaced concentric tracks arranged thereon. The disks are rotated by a spindle motor at a substantially constant rate of, typically, several thousand rpm (revolutions per minute). One or more read/write heads hover over the disks and either create magnetic domains in elemental areas called bit cells during write operations, or else detect the magnetic domains previously recorded in the bit cells during read operations. Magnetic domains are defined as the field of magnetic flux located between two flux transitions.
A major functional element of a magnetic disk data storage system is the data channel, which includes a write channel and a compatible read channel. The write channel receives binary source data from the host, encodes it, and records it as a series of magnetic flux transitions onto one or more disks of the HDD. The read channel retrieves the data from the disks, decodes it, and supplies it back to the host. Encoding and decoding are collectively known as the “ENDEC” function of the data channel.
The efficiency of a given ENDEC is measured by the Code Rate, indicated by the fraction N/M, where N bits of binary source data received from the host are recorded onto a disk utilizing M bit cells. Low code rates (those less than unity, or 1) cause inefficient use of the available disk space, which in turn generally means a high dollar cost per megabyte of storage space. High code rates result in more efficient use of the available disk space with attendant lower cost per megabyte of storage space.
It is an axiom within the computer industry that there is no such thing as enough memory and disk storage space. Consequently, magnetic storage system manufacturers continuously strive to increase the capacity of the disk drives they produce in order to offer the benefit of lower cost per megabyte of storage space to their customers.
Generally, the capacity of an HDD can be increased in three ways. Firstly, it can be increased by adding disk area, which is done by raising either the number or the diameter of the disks within the HDD unit. However, as the number of disks increases, the rotary drive force of the spindle motor used to rotate them needs to be raised. This introduces the problems of increased power consumption, heat, and noise generated within the HDD. On the other hand, increasing the diameter of the disks results in a physically larger HDD unit. While larger dimensions are a feasible option for disk drives intended for mainframe computers, they are not a viable option for disk drives intended for desktop, laptop, or handheld computers. Cost is also a significant issue when adding disk area.
Secondly, the capacity of an HDD can be increased by raising the recording density of the disks therein. Typically, this is done by increasing the linear bit density and/or the track density. Linear bit density refers to the number of elemental areas (bit cells) arranged per inch on a circular track of the disk, and it is increased by packing the bit cells closer together. However, when this is done, there results the problem of inter-symbol interference causing errors in the readback signal during a read operation. Track density refers to the number of tracks arranged per inch of the disk, as viewed in a radial direction, and it is increased by reducing the width and pitch of individual tracks. However, when the tracks are altered in this way, crosstalk between adjacent tracks becomes a significant problem and readback errors may result. Efforts to increase the recording density of HDDs have been ongoing, requiring very high expense and considerable engineering effort.
Thirdly, the capacity of an HDD can be increased by using a more efficient data encoding and decoding method. Encoding is the process used to convert binary source data received from the host into code data that can be stored on the disk drive. Code data is a series of magnetic flux transitions that occupy the bit cells on a disk and which represent the source data that was received from the host. An efficient encoding method is one in which there are fewer flux transitions occupying the disk than there are data bits received from the host. Several examples of prior art encoding methods are listed in the table below. Encoding methods are often called Codes, Code Types, or Modulation Codes.
TABLE 1CodeRateU.S. Pat. No.Issue DateAuthor(s)Code Type(N/M)3,374,475Mar. 19, 1968GaborGabor2/33,685,033Aug. 15, 1972Srivastava, etGCR 4/54/5al.4,323,931Apr. 06, 1982Jacoby, et al.3PM3/24,488,142Dec. 11, 1984FranaszekRLL 1, 72/34,506,252Mar. 19, 1985Jacoby, et al.Ternary3/25,422,760Jun. 06, 1995Abbott, et al.GCR 8/98/95,631,887May 20, 1997Hurst, Jr.PWM5/25,818,653Oct. 06, 1998Park, et al.QAM/PSK8/35,844,507Dec. 01, 1998ZookGCR 16/1716/176,032,284Feb. 29, 2000BlissTCM8/56,212,230Apr. 03, 2001Rybicki, et al.PPM8/3
It should be noted that codes have several characteristics in addition to code rate that are important to consider when making a determination of a particular code's usefulness. These characteristics are termed Figures of Merit. U.S. Pat. No. 4,928,187 issued May 22, 1990 to Rees discloses the Figures of Merit for several significant codes. Figures of Merit that are considered advantageous are a high data density or code rate, a low frequency ratio, a large recovery window, and a low error propagation distance.
Data density or code rate indicates the relationship between the number of binary source data bits N received from the host, and the number of bit cells M used to represent that data on the disk. A high data density is anything greater than unity, or 1.
The recovery window indicates the time period in which a given bit cell is sampled for valid data. Larger recovery windows reduce the possibility of readback errors. A recovery window bracketing an entire bit cell is most favorable.
The frequency ratio indicates the overall bandwidth requirement of the code under consideration. Frequency ratios of less than two are considered best.
Error propagation is a Figure of Merit that indicates how significantly a misread data bit will affect subsequent data bits read back from the storage medium during a read operation. This Figure of Merit tends to be sacrificed the most in the quest for maximum data density. Error propagation distances of less than two bit cells in duration are considered good.
Attempts have been made to develop modulation codes with progressively better Figures of Merit. Nevertheless, all the encoding and decoding methods heretofore known suffer from a number of disadvantages:
(a) Increasing the storage capacity of a magnetic disk storage system commonly requires physical alterations to the magnetic disk medium, read/write heads, or the mechanical interface therebetween. Prior-art alterations include thin-film recording surfaces, flying magneto-resistive read heads, higher disk rotational speeds, voice-coil-driven actuator arms, and smaller magnetic domains. These alterations come at great expense and considerable engineering effort.
(b) Prior-art encoding and decoding methods with code rates greater than unity commonly sacrifice other Figures of Merit in order to achieve the greater code rate.
(c) No prior-art encoding and decoding methods for magnetic HDDs have a code rate (N/M) greater than 3/1.
(d) Achieving high storage capacity in prior-art HDD designs commonly required using a plurality of disks. These disks are the greatest single expense in an HDD design and using a plurality of them results in a very expensive HDD unit.
(e) To achieve a level of increased performance, magnetic disks are commonly rotated at several thousand rpm, which requires the use of exotic disk materials to prevent the disks from shattering due to the high centrifugal forces generated.
(f) Achieving high storage capacity in prior-art HDD designs commonly required creating a disk drive with a large physical size, resulting in excess material usage, excess cost, and a greater negative impact on the environment.
(g) Achieving high data throughput (bits recorded and retrieved per second) in prior-art disk drives commonly required a high rotational speed. This is another contributor to the requirement of using exotic and expensive disk materials.
(h) There are physical limits to the maximum capacity of prior-art disk drives because of constraints imposed on a drive's physical dimensions.
(i) Increasing bit density has commonly involved simply packing the magnetic bits closer together. But this results in inter-symbol interference (ISI) when the bits exceed a certain proximity limit. This approach also necessitates sensitive read heads and advanced detection schemes to reliably detect the weaker magnetic domains associated with densely packed magnetic bits.
(j) In an effort to maximize total data storage capacity, prior-art HDD encoding and decoding methods typically sacrifice bit error rates or channel frequency characteristics in order to achieve the primary goal of maximum storage capacity. However, in systems such as these the overall performance, when taken as a whole, is not improved over other encoding and decoding systems of the art.
(k) Encoding and decoding methods have become increasingly costly and complex in an effort to pack as much data as possible onto a magnetic disk. Improvements in cost per megabyte have come as a result of mass production and better manufacturing tolerances rather than as a result of efficient encoding and decoding methodologies.
(l) Increasing the storage capacity in prior-art HDDs has almost always been achieved by packing the magnetic bits closer together on the disks. However, this has the undesirable result of increasing the read channel frequency and associated noise of the channel which requires complex compensation methods.